Steel railway cross ties have been used for over a hundred years. Cross ties made from steel have always had to compete against cross ties made from other materials such as timber or reinforced concrete. In the early part of the 20th century railway companies began treating timber cross ties with creosote. The creosote treatment improved the service life of timber cross ties and some railway companies began to favor using creosote treated timber cross ties over steel cross ties.
Current economic factors, including a declining cost differential between timber and steel, have made many railway companies reconsider using steel cross ties as an economically viable alternative to timber cross ties. This is particularly true when railway companies consider the longer service life of steel cross ties and the totality of costs over the entire service life of the cross ties. Steel cross ties also have a number of performance advantages over timber cross ties and reinforced concrete cross ties.
Some of the benefits and advantages of steel cross ties alluded to above are set out below:
a. Steel cross ties are lighter than concrete or timber cross ties so more steel cross ties can be shipped on a 100 ton capacity rail car, thereby reducing transportation costs. PA1 b. Steel cross ties have a lower profile than concrete or timber cross ties, so steel cross ties require approximately 500 to 700 cubic yards less ballast per mile. PA1 c. Because steel cross ties are lighter than concrete or timber cross ties and requires less ballast, this reduces the dead load that must be supported by bridges and foundations for railway tracks. PA1 d. Steel cross ties have a larger footprint than either concrete or timber cross ties, so steel cross ties can be spaced at greater center to center distances than timber cross ties while allowing greater load spreading capabilities. PA1 e. Steel cross ties have a better derailment survivability record compared to concrete or timber cross ties. PA1 f. Steel cross ties are not prone to deterioration like timber or concrete cross ties around fuelling facilities. PA1 g. Steel cross ties are more forgiving compared to timber or concrete cross ties under adverse conditions such as center bound track or poor surface conditions where surfacing programs have been cancelled or delayed. PA1 h. Steel ties require less material handling than timber ties with no tie plates, anchors or spikes to distribute. PA1 i. Steel cross ties can be handled manually or with existing maintenance equipment and tools. PA1 j. Steel cross ties can be installed with a Track Renewal Machine or mechanized tie gang. PA1 k. Steel cross ties have a longer service life than timber cross ties. PA1 l. Steel cross ties can be recycled.
Despite all of the above-noted benefits and advantages, a longstanding drawback of steel cross ties is that they can not be directly substituted for timber cross ties where there are electrical signal systems which use track electrical circuits. Approximately twenty percent of the total track mileage in North America has signal systems using track circuits. A track circuit uses the stock rails as the conductors and the train wheels and axles as the switch for activating signals when shorted. Unlike timber cross ties, steel cross ties conduct electricity. Therefore, steel cross ties installed on tracks using track circuits must be electrically insulated to prevent the cross ties from completing the track circuit.
One method of electrically insulating steel cross ties is to use a sandwich plate construction on top of the steel cross tie. According to this method, an insulating layer is sandwiched between the top of the steel tie and a protective load-distributing cover plate. The stock rails are affixed to the cover plate, and the cover plate is attached to the cross tie by an electrically insulated fastener. In a common arrangement, one set of bolts with electrically insulated sleeves are used to hold together the steel cover plate, the electrical insulating layer, and the steel cross tie. A second set of bolts mounts the clips and other hardware used for anchoring the stock rail to the cover plate.
A problem with using a sandwich plate construction is that the cover plate needs to be thick enough to carry the load without warping or tearing. The cover plate adds substantially to the weight and cost of this method of electrically insulating steel cross ties. Also, the plates and bolt hole configurations have to be custom fabricated for the particular layout of each turnout. Finally, if existing uninsulated steel cross ties are to be converted to electrically insulated steel cross ties, the conversion process using sandwich plate construction is complicated. Riser plates must be machined off, and the existing bolt holes in the steel cross ties need to be filled and new holes drilled or punched.
Another method of electrically insulating steel cross ties is to surround the stock rail with an insulating element where it is in contact with the cross tie. An example of this solution is disclosed by Wandrisco in U.S. Pat. No. 4,061,270 ("Wandrisco"), issued to United States Steel Corporation on Dec. 6, 1977. Wandrisco teaches a saddle-type insulating element for use with a steel cross tie which permits the stock rail to be used as an on-line signal control conveying device.
There are also problems associated with the solution taught by Wandrisco. First, this approach, like the sandwich plate arrangement, adds an electrically insulating layer between the stock rails and the cross ties. The additional thickness of the electrically insulating saddle may require non-standard clips and rail braces which can accommodate the extra insulating layer.
Another problem with saddle-type electrical insulators is that a large number of specially shaped saddles would be required to electrically insulate the cross ties used for a turnout. For example, the cross ties underneath the frog and switch areas would require a large number of insulating saddles with varying widths or shapes to accommodate the close proximity of the stock rail with other components and mounting hardware. The need for a variety of shapes and sizes of insulator pieces requires railway companies to keep a larger inventory of spare parts on hand for maintenance and new construction.
A problem with any electrically insulated steel cross tie which uses an insulating layer between the stock rail and the cross tie is that the insulating layer and protective plates, if any, increase the overall depth required to install the cross tie and the additional layers. Depending on the thickness of the added layers, this could complicate the conversion of an uninsulated switching area to an electrically insulated switching area.
Accordingly, there is a need for an electrically insulated steel cross tie which does not insert insulating layers between the stock rails and the cross ties, thereby avoiding many of the problems associated with sandwich plate arrangements and saddle-type insulators such as the ones described above.